I bought my 3500 in anticipation of buying a 5th wheel in the near future, and pulled my 7,000# TT in the meantime without the WD hitch since I really didn't need one. It pulled fine, but on a few of our roads with off camber sections and frost heaves, the trailer did move around a bit.
Since I had the hitch (Equal-i-zer with 4 point sway control) I hooked it up, and the affect was to eliminate the wiggles now and then, and to stop the truck from porpoising through the worst frost heaves. Neither was a real issue, but the fact is that they were there.
Bottom line is that even though I don't need it, it does make a difference that is noticeable, and if you have one, why not use it?